\input pstfig
\vrule height 15em width 0pt
$
\thinline
\pnode(0,0){O}
\psaxes[labels=none,ticks=none,arrowlength=5,arrowinset=0.2,arrowsize=1.8pt]{->}(0,0)(-3,-3)(4,3.5)
\uput[-45](4,0){x}
\uput{1ex}[0](0,3.5){y}
\uput[-135](O){O}
\thickline
\pnode(3,2){A}
\pnode(A|O){B}
\pspolygon(A)(O)(B)
\thinline
\markang{->}(B)(O)(A){0.9}
\uput{0.4}[18](O){\alpha}
\noneline
\pcline(O)(A)
\Aput{r}
\pcline(O)(B)
\Bput{x}
\pcline(A)(B)
\Aput{y}
\uput[20](A){P(x,y)}
$
\bye

